The present invention relates generally to a measuring device, and more particularly to a probe for conducting measurements in a substrate into which the probe is driven.
Still more specifically, the invention is concerned with a measuring probe for measuring skin friction and peak pressure in a substrate.
Measuring devices of the type in question are widely used for determining the characteristics of soil strata, especially those located relatively far from the surface. They employ a probe which is forced into the substrate, that is into the soil in this case, and which is provided with measuring arrangements. The user of such a probe can make deductions concerning the character of the soil strata, the consistency and the layering, from the force required to push the probe body through the soil, i.e., from the resistance of the soil to penetration. The total force acting upon the pressure measuring device of the probe is composed of the force acting lengthwise of the probe upon the tip at the leaading end thereof, and the skin friction acting upon surface portions of the probe. When both the local peak pressure and the local skin friction are jointly known, conclusions may be drawn concerning the soil characteristics, for example concerning the change of the soil characteristic from a looser to a more compact stratum, or vice versa.
Measuring devices for measuring the peak pressure that develops during the insertion of such probes are known. They may, for example, have a thin-walled cylindrical member the elastic compression of which--resulting from soil resistance to the insertion of the probe--serves as a measure of the resistance acting upon the tip of the probe. A sensor is used which measures the degree of compression and delivers an electrical signal which is supplied to a receiver. It is also known in the prior art to provide an arrangement measuring the total force required to push the probe into the soil. In other words, these devices measure the total force required to insert the probe and the peak pressure which develops, and by taking the difference of these two forces they derive a value for the skin friction acting upon the total probe which is composed of the probe body and the probe-inserting rods or the like.
This manner of determining the peak pressure and the skin friction has various disadvantages, not the least of which is the fact that it is complicated since skin friction is not measured directly, but as the difference of two values. This also means that the measurement of skin friction is inevitably subject to errors of an unacceptable magnitude. The measuring arrangement for measuring the peak pressure in these devices is located within the tip of the probe body. whereas the skin friction that is determined by taking the difference between the total inserting force and the peak pressure includes the skin friction which acts not only on the probe body but also upon the inserting rod or rods, so that the measured skin friction value is not the value applicable to the probe body alone, as it should be, but is increased by the value of the skin friction acting upon the inserting rod or rods; it is therefore inaccurate.